This invention pertains to apparatus for automating the administration and management of intravenous regional anesthesia (IVRA) for both upper and lower limbs. IVRA is an alternative to general anesthesia for limb surgery. IVRA has proven to be a simple and useful technique for satisfactorily anesthetizing the upper limb and is potentially well suited for greatly expanded utilization in surgery of lower limbs and in outpatient settings. In these settings, which are rapidly increasing in number worldwide, there is a large and unmet need for a rapid, simple, safe and reliable technique for establishing limb anesthesia. However, significant practical problems with the technology of IVRA in the prior art, considerable variations in skill involving the manual administration of IVRA, and lingering concerns over the potential toxicity of certain IVRA agents, particularly for lower limbs, have greatly limited the acceptance of this promising technique.
IVRA is an anesthetic technique which requires the use of a surgical pneumatic tourniquet ststem. Surgical pneumatic tourniquet systems are frequently used on the upper and lower limbs to help maintain a bloodless operative field by regulating the maximum pressure applied to the limb by an encircling cuff at a pressure sufficient to stop arterial blood flow past the cuff for the duration of a surgical procedure. During operations performed under IVRA, the pneumatic tourniquet serves an additional role of preventing liquid anesthetic agent introduced into the veins in the limb distal to the cuff from flowing proximally past the cuff and out of the limb into the circulatory system. An insufficient pressure in the tourniquet cuff soon after introduction of the liquid anesthetic agent into the limb may result in the anesthetic agent entering the circulatory system in a high concentration, which can cause serious adverse reactions such as cardiovascular collapse, respiratory depression, epileptic seizures or even death.
IVRA is typically administered as follows. The limb is first exsanguinated, often by wrapping the limb with an elastic bandage, beginning distally and wrapping tightly towards the heart; after exsanguination, a tourniquet cuff is applied proximal to the operative site and inflated to a predetermined cuff pressure. The elastic bandage is removed and a liquid anesthetic agent such as lidocaine mixed with sterile saline is introduced into a vein in the limb through an intravenous cannula. The anesthetic liquid remains in the veins in the limb distal to the cuff as long as the tourniquet is inflated to a sufficient pressure. Premature release of the agent shortly after introduction, as well as leakage of the agent under the cuff during introduction or during surgery, are serious and recognized hazards associated with prior art devices used for IVRA.
Surgical tourniquet systems of the prior art typically include an inflatable cuff for encircling a limb and an automatic pressure regulator for maintaining the pressure to which the cuff is inflated near a reference pressure selected by an operator or determined automatically. Recently, research and clinical investigations were completed with an improved surgical tourniquet system which incorporated a novel biomedical pressure transducer disclosed by McEwen in U.S. Pat. No. 4,869,269. The novel transducer was positioned underneath the cuff to measure the pressure distribution applied by the cuff to the limb at a number of discrete locations relative to the cuff. Experiments revealed that normal variations in technique used to apply the cuff to the limb were found to vary the maximum pressure actually applied to the limb by 50 percent or more, in comparison to the regulated bladder pressure of the cuff. This variability has important implications for IVRA, especially with regard to the potentially serious and recognized hazard of premature release or leakage of the liquid anesthetic agent proximally past the cuff and into the general circulation.
There are also specific hazards associated with the use of prior art tourniquet systems for IVRA because there has been little or no monitoring of maximum pressure actually applied to the limb by the cuff, the pressure at which anesthetic liquid is injected, and the increased pressure in the veins due to introduction of the anesthetic liquid. Injection of the anesthetic liquid at a high pressure has been shown to lead to excessive pressures in the veins distal to the tourniquet cuff, which in some cases has led to anesthetic liquid flowing past the cuff and into general circulation. Similarly, because of variations in cuff types and application techniques, despite an appropriately chosen cuff inflation pressure, the maximum pressure actually applied to the limb by the cuff may be significantly less than the regulated pneumatic bladder pressure indicated on the tourniquet pressure display; in this case, injection of anesthetic liquid at only a moderate pressure may increase the pressure in the veins above the pressure actually applied by the cuff so that anesthetic liquid flows proximally past the cuff. Both of the above-described situations can lead to an ineffective regional anesthesia in general, and to increased hazards, including cardiac arrest and death in a small number of reported cases.
Another hazard associated with the use of IVRA is related to the release of the anesthetic liquid from the limb upon completion of the surgical procedure. Although cyclical cuff deflation and re-inflation protocols requiring manual operation of tourniquets for predetermined periods and pressures at the completion of surgery have been recommended in the prior art, no apparatus for automatic cycling of cuff deflation and inflation has been implemented in the prior art, and in many clinical situations the anesthetic liquid is simply released from the limb in a single bolus by fully deflating the tourniquet cuff at the completion of surgery. The reported incidence of toxic reactions to post-operative release of the anesthetic liquid from the limb is approximately 2 percent, the majority of the reactions being manifested typically within one minute of tourniquet release as minor, temporary disturbances of the central nervous system (CNS) such as giddiness, dizziness, or tinnitus (ringing in the ears) and in some cases minor bradycardia (reduction in heart rate). More serious phenomena, such as muscle twitching, convulsions, and loss of consciousness have been reported at an incidence of approximately 0.2 percent in cases in which the anesthetic agent was lidocaine. Convulsions have been reported in many cases in which bupivicaine has been used. Nothing in the prior art known to the applicant has suggested control of the tourniquet apparatus based on monitored physiological changes arising from to the release of anesthetic liquids, despite the present availability of techniques for non-invasively observing physiological changes associated with the reported symptoms of CNS toxicity.
A related hazard is that the administering anesthetist generally is unable to detect or monitor any possible leakage of the anesthetic agent during a surgical procedure involving the use of IVRA. The prior art does not describe, mention or suggest apparatus for monitoring the concentration of anesthetic liquid in blood proximal to the cuff, or distal to the cuff, during IVRA. The recent development of transcutaneous, intra-arterial and intravenous transducers for estimating anesthetic concentration by a variety of direct and indirect approaches has made this feasible.
Another hazard recognized in use of IVRA is related to the volume of liquid anesthetic introduced into the limb, particularly in the case of procedures performed on the lower extremity in which a larger volume of liquid anesthetic may be required. In the prior art, this factor, combined with concerns over potential toxicity has been the primary limitation preventing extension of IVRA to many lower limb procedures. Protocols described in the prior art have largely specified which agents, volumes, and concentrations for given clinical situations are appropriate. Despite the potentially larger toxic hazards associated with larger volumes of anesthetic agent, the applicant in unaware of any recommendation in the prior art concerning IVRA which suggests that the volume of liquid anesthetic delivered be a function of the level of anesthesia produced in a portion of the limb distal to the cuff, even although clinical experiments were conducted in the prior art in which the level of anesthesia during IVRA was related to conventional electromyographic determinations of motor nerve conduction velocity.
In summary, IVRA has proven to be a simple and useful technique for satisfactorily anesthetizing the upper limb and is potentially well suited for greatly expanded utilization in surgery of lower limbs and in outpatient settings. However, significant practical problems with the technology of IVRA in the prior art and lingering concerns over the potential toxicity of certain IVRA agents have greatly limited the acceptance of this promising technique. Despite potential complications associated with leakage of anesthetic liquid proximally under the cuff, the applicant is unaware of any apparatus in the prior art which alerts the administering anesthetist by means of an alarm, or which prevents leakage of anesthetic liquid by means of anesthetic control means, in response to monitoring and estimation of the maximum pressure actually applied to the limb by the cuff and the pressure of the anesthetic liquid introduced into the limb distal to the cuff. Prevention of toxic reactions to post-operative anesthetic release have not, to the applicant's knowledge, been addressed in the prior art by controlling the tourniquet in response to the concentration of the anesthetic liquid in the patient's body proximal to the cuff. The applicant is also unaware of any apparatus described in the prior art in which anesthetic control means responsive to the level of anesthesia introduces sufficient anesthetic liquid to establish a level of anesthesia near a predetermined level, thereby preventing the introduction of an unnecessary and potentially hazardous volume of drug.
An object of the present invention is to provide apparatus for assisting in the administration of IVRA, including a transducer for producing a signal representative of the maximum pressure applied to the limb by a cuff encircling the limb, a transducer for producing a signal indicative of the anesthetic liquid pressure within a vein of the limb distal to the cuff, and an alarm which is activated when the difference between the applied pressure signal and the anesthetic liquid pressure signal is less than a preassigned safety limit, to alert the administering anesthetist and thereby prevent inadvertent leakage of anesthetic liquid under the cuff. A related object of the present invention is to include apparatus for stopping the introduction of anesthetic liquid when the alarm is activated.
Another object of the present invention is to provide apparatus for administering anesthesia to a portion a limb distal to a cuff, including a transducer for producing a signal representative of a pressure applied to the limb by a cuff encircling the limb, means for delivering the anesthetic liquid into a vein in the limb and for producing a delivery pressure signal indicative of the delivery pressure, and an alarm which is activated when the difference between the applied pressure signal and the delivery pressure signal is less than a preassigned safety limit to alert the administering anesthetist and thereby prevent leakage of anesthetic liquid under the cuff.
A further object of the present invention is to provide apparatus for the containment and release of anesthetic liquid from a limb encircled by a pressurizing cuff which restricts flow of anesthetic liquid proximally past the cuff, including means for controlling the pressure applied by the cuff to the limb and means for estimating the concentration of the anesthetic liquid in the patient's body proximal to the cuff. A related object of the present invention is to provide control means which restricts flow proximally past the cuff to keep the concentration of anesthetic liquid below a preassigned concentration limit so that CNS toxicity is avoided. A further related object is to include an alarm which is activated if the estimated concentration exceeds a preassigned safety limit, to warn of impending toxicity.
Another object of the present invention is to provide apparatus for establishing a desired level of anesthesia in a portion of a limb distal to a pressurizing cuff which encircles the limb, including means for estimating the level of anesthesia in the portion of the limb distal to the cuff, and anesthetic control means responsive to the estimated level of anesthesia which activates anesthetic delivery means to introduce sufficient anesthetic liquid to result in a level of anesthesia near a predetermined level, thereby preventing the introduction of an unnecessary and potentially hazardous volume of liquid anesthetic agent. A related object of the invention is to include an alarm which is activated when the level of anesthesia exceeds a preassigned safety limit.